The present invention relates to a communication system for digital data transmission via subscriber cables, and more particularly to a DC termination circuit for use in such a communication system.
Where data terminal equipment (DTE) and subscriber cables are connected to each other at contact points to achieve data transmission via the cables, the surfaces of the contact points are known to sometimes become oxidized and gather rust with the lapse of time. This rust would in no way affect the transmission of analog signals, such as speech signals, via the subscriber cables. However, if data are directly transmitted via the cables, an increase in resistance due to the rust will invite data errors and thereby obstruct transmission of accurate data. By the prior art, in order to prevent the occurrence of such rust, a DC termination circuit consisting of a resistor is connected to the contact points, and a DC current known as a sealing current is supplied to this resistor from the central station.
The conventional DC termination circuit referred to above, merely consisting of a resistor for termination, involves the problem that, in the event of communication trouble or the like, it is difficult to check from the central station to determine whether there is any abnormality on the cable. More specifically, abnormality checking is tried from the central station by measuring the resistance and the electrostatic capacitance of the cables. The resistance can be measured without removing the DC termination circuit; the capacitance cannot be so checked. Accordingly, there is the great trouble, and consequently the inconvenience for practical use, of having to disconnect the DC termination circuit every time the capacitance is to be measured.